Flashcards in Structure and function of the eye Deck (58)
Describe the structure of the retina
Retinal pigment epithelium; provides nutrients and removes metabolic debris from the photoreceptors
Describe the layers of the neuroretina
Outer layer: photoreceptors capture light
Horizontal cells modulate the signal from the photoreceptors
Middle layer: bipolar cells regulate the sensitivity
Inner layer: retinal ganglion cells pass the signal from the eye to the brain
How can you visualised the macula?
Use optical coherence tomography
Describe rod photoreceptors and their location
Long outer segment with photo-sensitive pigment
Needed in night vision, detecting motion, peripheral vision (scotopic vision)
Slow response to light
100 times more sensitive than cones
Highly concentrated outside the macula, 20-40 degrees away from the fovea
120 million rods
Describe cone photoreceptors and their location
Less sensitive to light
Detailed daylight and colour vision (photopic vision)
Only concentrated within the macula, 0 degrees from the fovea
60 million cones
What is the frequency of the human visual spectrum?
What are the rod and cone photopigments?
Rod: 498nm, blue-green
Cone: S (blue), M (green), L (red)
What is the most common colour vision deficiency called and caused by?
Deuteranomly (red-green colour blindness); M cone sensitivity shifts towards L-cone so causes red-green confusion
What is meant by dichromatism and monochromatism?
Dichromatism; absence of one of the cone photopigments
Monochromatism; complete absence of colour vision
What is meant by anomalous trichromatism?
When colour blindness is caused by a shifted peak
How is red-green colour blindness identified?
Ishihara colour perception test
What is dark adaptation?
When photoreceptors become more sensitive to light in the dark
Cones; adapt within 7 minutes
Rods; more sensitive so adapt in 30mins, must generate rhodopsin
What is light adaptation?
Adaptation from dark to light
Takes 5 minutes
Bleaching of photopigments
Rod/cone function inhibition
What is pupil adaptation?
Constriction of pupil in response to light
What is the index of refraction formula?
Index of refraction= speed of light in a vacuum/speed of light in medium
Difference between convex and concave lens?
Convex: light rays converge onto focal point
Concave: light rays diverge
Which structures in the eye help concentrate light?
Cornea and lens focus light rays onto the retina
Which structures in the eye regulate light entry?
Pupil and pigmented uvea (absorbs excess scattered light within the eye)
What is meant by emmetropia?
Eyes are relaxed and lens is focused on object far away. Light rays are almost parallel and rays are focused on the retina without effort ( no accomodation).
What is meant by ametropia?
When there is axial length and refractive power mismatch. Parallel light rays do not fall on the retina.
Near-sightedness (myopia), Far-sightedness (hyperopia)
What is the pathology of myopia?
Light rays converge at a point anterior to the retina
Eyeball is longer (axial/globe length)
Excessive refractive index
Squint to improve visual acuity
Correct using concave lenses
What is the pathology of hyperopia?
Light rays converge at a point posterior to the retina
Short globe/axial length
Inadequate refractive power (flat corneal surface)
Asthenopic/eye strain symptoms; eye pain, headache in frontal region, burning
Correct using convex lenses or intraocular lenses
What is Amblyopia?
Lazy eye, uncorrected hyperopia of more than 5 diopters
What is astigmatism?
Parallel rays converge in two lines rather than one.
Cornea is of oval not spherical shape.
Refractive power varies along different planes.
Asthenotopic symptoms; blurred vision, spinning
Treat using cylinder lenses or rigid contact lenses or surgery.
What is accommodation and how does it occur?
Contraction of ciliary muscles, relaxation of zonule fibres.
Absence of zonular tension causes lens to relax into convex shape due to elasticity.
Increases refractive power of the lens
Mediated by CNIII
Describe the near response triad
1. Pupillary mitosis; increases depth of field
2. Convergence; medial recti of both eyes
3. Accomodation; makes lens thicker to increase refractive power
What is presbyopia?
Naturally occurring (40+ yrs) loss of accomodation
Due to hardening of crystalline lens
Loss of visual acuity in near objects
May still be emmetropic for far objects
Treat using positive convex/converging lens to increase optical power